1. Field of the Invention
The present invention relates to an orthogonal transformation encoder to be used for making the compression ratio in the case of high-efficiency coding of a video signal.
2. Description of the Related Art
The bit rate reduction technology (or high-efficiency coding technology) for reducing the data quantity of an original video signal for the purpose of long hour picture recording is used to constitute a recording/reproducing apparatus (for example, a VTR, a video disc, etc.) for recording/reproducing a video signal. The orthogonal transformation coding is a process in which a video signal is divided into blocks and frequency analysis is performed on every block, so that the quantity of data to be transmitted (or to be recorded) is made smaller as the frequency of components is higher. That is, a visual character is utilized such that it is more difficult to detect deterioration as the frequency of components is higher.
FIG. 1 is a block diagram showing a conventional orthogonal transformation encoder for performing such orthogonal transformation coding. The operation of the orthogonal transformation encoder will be described hereunder.
The conventional orthogonal transformation encoder is constituted by: a conventional input terminal 1 for receiving a blocking signal of a video signal; an orthogonal transformation circuit 2 for two-dimensionally transforming the blocking signal; a scanning circuit 3 for performing rearrangement of the orthogonal transformation coefficients obtained from the orthogonal transformation circuit 2; a coding circuit 4 for coding the orthogonal transformation coefficients outputted from the scanning circuit 3; and an output terminal 5.
The orthogonal transformation circuit 2 is constituted by a horizontal orthogonal transformation circuit 21 for performing orthogonal transformation in the horizontal direction; a rearrangement memory 22 for rearranging the horizontally arranged coefficients of the orthogonal transformation into the vertical direction; and a vertical orthogonal transformation circuit 23 for performing orthogonal transformation in the vertical direction.
The coding circuit 4 is constituted by a quantizer 41 for quantizing, with a given step width, the coefficients of the orthogonal transformation, a quantization selection circuit 42 for selecting so that the quantizer has a step width suitable for arranging a data quantity to be a desired value, and a coding device 43 for coding the quantized data outputted from the quantizer 41.
With respect to the frequency of occurrence of a coded word, the coding device 43 performs variable length coding for assigning a coded word which is made shorter in length (smaller in data quantity) as the frequency of occurrence of the coded word is higher. Accordingly, the quantization selection circuit 42 calculates the data quantity to be obtained as a result of the variable length coding to whereby make a selection so that the quantizer has an optimum step width.
For the coding for two-dimensional orthogonal transformation, two-dimensional arrangement from low frequency components to high frequency components, called zigzag scanning, is preferable. That is, the lower frequency components are made more important than the higher frequency components because the lower frequency components give a visual influence greater than that of the higher frequency components. To this end, in the coding circuit 4, the step width is made finer successively starting from a condition where the quantization is performed with the most largest step width to a condition where the data quantity is made to be a desired value.
The above-mentioned construction has the following problems.
A first problem is that in the orthogonal transformation circuit 2, it is possible to perform the orthogonal transformation only by frame in the case a video signal to be subjected to the orthogonal transformation is transmitted by frame. That is, when a video signal which moves between fields is inputted, intra-frame two-dimensional orthogonal transformation is performed regardless of a large field correlation. Accordingly, intra-field moving components appear in the high frequency components in the vertical direction on the orthogonal transformation coefficients to thereby extremely impair the data quantity reducing efficiency.
A second problem exists in that in the encoding circuit 4, the same quantization selection is made evenly to a signal block which contains only small-amplitude frequency components and a signal block which contains large-amplitude frequency components.
In view of the data quantity through coding, signal blocks may be classified into two kinds of blocks. That is, blocks of a large data quantity include blocks containing large-amplitude frequency components and blocks of a small data quantity include blocks containing only small-amplitude frequency components. If the same step width is assigned to those two kinds of blocks, the blocks containing large-amplitude frequency components are not deleted even by the large step width because of the large amplitude thereof. On the other hand, most of the coefficients of the later blocks containing only small-amplitude frequency components is deleted (that is, the coefficients are assigned to "0"). This means that with respect to blocks having important information though they are blocks on a flat picture screen, the important information is lost (remarkably, for example, in the trees and plants or sky in the background).